Research is continuing on experimental and theoretical studies of carrier- and channel-mediated transport of ions across lipid bilayer membranes with the broad objective of understanding, at a molecular level, the selectivity and permeation mechanisms of model carriers, model channels and nerve. Experimental and theoretical studies will be carried out for well defined carriers (typically synthetic cyclic polypeptides, depsi peptides, and macrotetralides) and channels (typically Valine Gramicidin A). These studies are directed toward ascertaining the essential similarities and differences between carriers and channels in their bioelectrical properties. Additionally, experimental and theoretical studies will be done of the influence on ion selectivity of the electronic and molecular structure of the binding site whether it be as a carrier or in a channel, as well as of the electronic and molecular structure of the bound ion. These studies, which will include kinetic as well as equilibrium considerations, should extend the basis, already laid by Hille and Armstrong, for using ions as "probes" of the structure of the Na ion and K ion channels of nerve, since it is possible in artificial systems to study systematically variation in the dimensions of the ion binding site as well as the electron-density (or "field strength") of the ion binding ligands in selected carrier molecules and channels.